1. Field of the Invention
The present invention relates to electrochemical devices such as batteries, fuel or photovoltaic cells and oxygen separators, catalysts, sensors, etc. and in particular to a current coupling (including collection or distribution) structure for such electrochemical devices.
2. Description of Related Art
A solid oxide fuel cell (SOFC) is an energy conversion or power generating device that produces direct-current electricity by electrochemically reacting a gaseous fuel (e.g., hydrogen) with an oxidant (e.g., oxygen) across a single cell of oxide electrolyte sandwiched between a cathode electrode layer and an anode electrode layer. The key features of current SOFC technology include all solid-state construction, multi-fuel capability, and high-temperature operation. Because of these features, the SOFC has the potential to be a high-performance, clean and efficient electric power source that is under development for stationary and mobile applications.
It is known that the principal losses in most solid state electrochemical devices occur in the electrodes and/or at electrode/electrolyte interfaces. It is also recognized that minimization of these losses, which arise from either concentration polarization or activation polarization or both, is crucial to the efficient operation of these devices. For example, minimization of these losses is central to obtaining high current and power densities in solid oxide fuel cells.
Under typical operating conditions, an SOFC single cell produces less than 1V. Thus, for practical applications, single cells are stacked in electrical series to build voltage. Stacking is provided by a component, referred to as an interconnect, that electrically connects the anode of one cell to the cathode of the next cell in a stack. Conventional SOFCs are operated at about 1000° C. and ambient pressure.
Costs of SOFC systems are still too high for the technology to be considered commercially competitive. Expense is primarily due to the poor performance of the SOFC stack. The focus of SOFC costs reductions programs are electrolyte fabrication, electrode microstructure, and interconnect design and materials. The first two challenges have been addressed, but interconnect design and materials still need to be improved.
Several processing techniques have been developed to produce thin, air-tight electrolytes of yttria stabilized zirconia with low resistance. Electrode microstructures that have low activation resistances are widely known and utilized.
One example of a SOFC single cell is a ceramic tri-layer consisting of an oxide yttria-doped or stabilized zirconia electrolyte (YSZ) sandwiched between nickel/YSZ for the anode and a strontium-doped lanthanum manganite (LSM) cathode connected to a doped lanthanum chromite interconnect. Typical and state-of-the-art single cells are based upon a porous composite cathode of Sr0.2La0.8MnO3(LSM)/8 m/o yttria-stabilized zirconia (8 YSZ), a porous composite anode of nickel/8YSZ, and a YSZ electrolyte and deliver power densities in excess of 2 W/cm2 at 800° C.
Improvements in interconnect materials and design are still needed. The interconnects of anode-support designs are based upon transition metals and develop oxide scales that impede current flow. It is only recently recognized that the design of the interconnect and single-cells in some designs does not efficiently collect current from the electrodes. The role and effect of current distribution in the electrode on system performance has not been addressed.
System performance is sometimes dominated entirely by current distribution losses in the electrodes. In fact, the resistive effects of the single cell can be negligible.
In contrast to anode-suported or cathode-supported electrolyte, Corning's solid oxide fuel cell (SOFC) design is based upon a thin, mechanically flexible electrolyte sheet as disclosed in U.S. Pat. No. 5,273,837. The electrolyte acts as the support for the electrodes and is punched with via holes for “through-the-electrolyte” interconnections as disclosed in U.S. Pat. No. 6,623,881. Unlike other SOFC designs where voltage is built by interconnection of separate electrolyte bearing elements such as cathode-supported tubes or anode-supported plates, the Corning design integrates the interconnect with the electrolyte to build voltage from multiple single cells arranged on a single electrolyte membrane.
Overall, the Corning design has the potential to deliver higher volumetric power density than other designs. Besides low cost of materials, the flexible electrolyte design with through-the-electrolyte interconnects can simultaneously solve the interconnect material problem and distribute current to (collect current from) electrodes.
Although cost is the ultimate determinant of commercial viability, performance is linked to cost within any design. Area specific resistance (ASR) is a commonly cited figure. of merit for fuel cells. The absolute slope of the plot of cell voltage vs. current density is defined as the area specific resistance of the cell (ohm-cm2).
Many factors contribute to ASR such as materials properties, processing conditions, and design geometry. Though not accounting for the effect of processing, the properties of most materials used to construct an SOFC are known. Performance and cost of a design can be predicted and optimized.
Activation polarization and resistance to oxygen ion transport are the primary contributors to internal resistance of the single cell. Theoretical fabrication and essential features of low internal resistance cells are known. It must be mentioned that concentration polarization can impact performance in certain situations, however, such effects are typically negligible except at current densities in excess of 5 A/cm2 or under fuel or oxidizer starvation conditions. At the next level of design, current is collected from the cathode into an interconnect pad, through the via to another interconnect pad, and is finally distributed throughout the anode of the strip cell of the Corning type.
Single cells should be designed to facilitate current distribution and collection in the electrodes. However, high power density of a well-designed single-cell can still be lost during current distribution. Distribution/collection losses become excessive when electrodes are too wide, too thin, or lacking in conductivity. This is especially true for LSM in the cathode. Conductivity of LSM is only 100 S/cm in comparison to 24,000 S/cm for nickel in the anode. Optimum theoretical electrode width for a 20 μm thick LSM cathode is less than approximately 1 mm to minimize power losses. In current practice, it is difficult to manufacture strip cell electrodes of that width. At present, a high conductivity (>10,000 S/cm) layer of a porous silver-palladium alloy that is about 10 μm in thickness is deposited on top of the cathode to facilitate current distribution. Such a fuel cell is described in U.S. Pat. No. 6,623,881 where the electrical conductors are relatively flat and made from silver-palladium alloys (e.g., 70% silver-30% palladium). Although the use of the flat silver-palladium electrical conductors disclosed in this patent application works well in most applications they can in some applications limit the durability of the fuel cell and may not meet cost requirements. Silver is volatile and mobile at normal operation temperatures of a SOFC.
One solution would be to replace silver with a more refractory noble metal current collector like gold. Material cost of the current collector per kilowatt can be estimated and related to cell ASR by the following equation 4000×ASR×p×d×t where p is cost of the current collector material in dollars per gram, d is the density, and t is the thickness required to achieve the designated ASR. The conductivity of gold is similar to silver, thus a 10 μm thick gold layer would also be desired. A performance target of ˜0.5 W/cm2 at maximum power corresponds to an ASR of 0.5 Ωcm2. This alone gives an estimated material cost of ˜200 dollars per kilowatt for the current collector that is excessive.
Therefore, a manufacturable interconnect design that does not rely upon silver and/or minimizes the use of precious metal in the cathode current collector is desirable. Solutions that eliminate silver also broaden the operation temperature range from 600-800 to 600-900° C. This is advantageous as thermal management constraints are eased somewhat and ASR can be lower at higher temperatures.
Therefore, there is a need for various SOFC design options that optimize performance (minimize ASR) under constraints of a given set of material properties, cost, and ability to manufacture while maximizing power output at the stack level. Such conditions to improve designs of single-cells in conjunction with the via and via/electrode contact include no silver, a fixed quantity of precious metal per single cell, a limited number of interconnects per single cell, manufacturable electrode width, use of oxide cathode current collectors, and shaped vias, etc.
In particular, SOFC designs that eliminate silver in general and specifically within the current collection structure used to distribute electrons through out the cathode has the following advantages:                1) Enhances operational system lifetime;        2) Enables operation at higher temperatures where specific power is higher; and        3) Eases constraints of temperature management during operation.        
Accordingly, there is a need for a fuel cell that utilizes electrically conductive current collectors which have a specific composition and/or a specific geometry that enhances the durability of the fuel cell, yet is cost-effective. This need and other needs are addressed by the fuel cell and electrical conductors of the present invention.